(1) Field of the Invention
The present invention relates to the field of fabricating semiconductor devices, and more particularly to anisotropically etching metal films formed on a semiconductor substrate of an integrated circuit for the fabrication of interconnects.
(2) Prior Art
In the manufacture of integrated circuits, it is often desired to etch certain areas of a metal film deposited on a semiconductor substrate to form metal interconnects between semiconductor devices. It is also desired to form metal interconnects having high aspect ratios for increasing the performance characteristics of the interconnects and for maximizing the device density of the integrated circuit. In the prior art, this has been accomplished through reactive ion etching of aluminum films wherein the ion-assisted etch process produces an anisotropic etch of the aluminum film to obtain the well-defined, high aspect ratios currently required in many of todays applications.
However, due to the increased speed and current density requirements for devices reaching submicron dimensions, the use of aluminum interconnects have become increasingly marginal. This is because the relatively high resistivity of aluminum as a metal, as seen in Table I shown below, hinders the performance and reliability of interconnects made from aluminum. This problem is particularly acute in current ultra-large scale integration (ULSI) devices which have relatively high aspect ratios of 2-3:1 or more. In addition, the present method for etching aluminum. interconnects also involves several drawbacks, including the formation of a native oxide on the aluminum film that is difficult to remove, the use of chlorine-based gasses that are either carcinogenic or highly toxic, and the formation of reactive compounds that degrade the pumping fluids used in the etch process.
TABLE I ______________________________________ RESISTIVITY OF METALS DEISIREABLE FOR USE IN INTERCONNECTS (in .mu..OMEGA.-cm) ______________________________________ Silver 1.6 Copper 1.7 Gold 2.4 Aluminum 2.65 ______________________________________
Due to the marginal performance and reliability of aluminum interconnects, some prior art methods have attempted to use copper instead of aluminum films since copper films provide a substantially lower resistivity and higher electromigration resistance. However, several problems in the formation of copper interconnects have rendered the prior art use of copper ineffective for interconnect applications. One of the problems with copper has been the difficulty in patterning the copper film. Although this problem was somewhat resolved through the use of selective deposition techniques, i.e. selective CVD or electrolysis plating, such techniques cannot be used in production because the presence of small defects or particulate contamination results in the loss of selectivity or the growth of large precipitates. For this reason, integrated circuit manufacturers have been trying to develop etching techniques for copper films that result in highly anisotropic and very selective etches.
Recently, it has been found that the anisotropic etching of copper using a chlorine-based gas chemistry in a reactive ion etch process can be achieved at submicron dimensions using elevated temperatures, i.e. around 250.degree. to 300.degree. C. Although the anisotropic etching of copper is a major break through, the high temperature requirement poses severe difficulties for plasma etch equipment manufacturers as well as process engineers in trying to develop a manufacturable etcher and process. This is because the volatile compound, copper chloride, produced in the high temperature etch process condenses on cold surfaces, thereby creating the problem of: copper chloride deposition in the vacuum lines, in the pumps and even on the unheated walls of the etch chamber. In addition, the high temperature requirement further prevents the use of a suitable masking material since most masks cannot withstand temperatures above approximately 120.degree. C.
Although the prior art methods for etching interconnects have found some success with the anisotropic etching of aluminum and copper, it is widely recognized that a method which permits the anisotropic etching of other highly conductive metals such as silver, gold and molybdenum would also be of great importance and utility.
Accordingly, it is an object of the present invention to provide a method for anisotropically etching a variety of metal films to obtain metal interconnects having very high aspect ratios and minimal undercutting.
It is a further object of the present invention to provide a method for anisotropically etching highly conductive metal films to form ULSI interconnects having a very low resistivity and high electromigration resistance in order to obtain ULSI interconnects that meet the increased speed and current density requirements.
It is another object of the invention to provide a method for anisotropically etching silver, copper, gold, aluminum and molybdenum films at low temperatures for interconnect applications.